Quantitative aspects of synaptogenesis in the visual system will be investigated from the number and distribution of photoreceptor synapses in the first optic neuropile of Musca. Studies will concentrate upon elucidating two major questions: 1) What are the developmental determinants of synapse number and spacing; 2) What influence has visual experience upon these determinants? Determination of synapse number will be examined amongst naturally occurring differences in innervation ratios between pre- and postsynaptic neurons and analyzed using quantitative EM sampling techniques. Synapse spacing will be analyzed at different developmental ages either from freeze-fractured presynaptic membrane or serial-EM of postsynaptic dendrites, to test the influence of one synaptic site upon synaptogenesis nearby and, consequently, to examine the relationship between synaptogenesis and dendritic morphogenesis. Using similar analyses, the effects of visual experience on synaptic population size and distribution will be examined following exposure to light and/or dark, either during development or in the adult. Qualitative aspects of synaptogenesis will examine possible altered synaptic classes formed either following laser-induced deletions of the postsynaptic neurons during development or in various mutants of Drosophila either lacking photoreceptor function or with the suspected absence of a class of feedback synapses or the suspected presence of an additional postsynaptic neuron. These will all be analyzed with quantitative- and serial-EM techniques with the aim of understanding the rules with which neurons normally combine at multiple-contact synapses (dyads, triads, etc.) from the novel combinations formed following perturbations. It is anticipated these studies will inform us about the regulation of synaptic population size, composition and distribution in visual neurons during development under eithr the influence of changes in cell number of differences in visual experience.